1. Field of the Invention
The present invention relates to a thermal print head, a method of manufacturing the same and a method of adjusting heat generation thereof. More specifically, the present invention relates to a thermal print head having a heat generation part consisting of a resistor which is prepared from polycrystalline silicon, a method of manufacturing the same and a method of adjusting heat generated from the heat generation part.
2. Description of the Background Art
FIG. 7 is a perspective view showing the overall appearance of a conventional thin film type thermal print head and FIG. 8 is a sectional view thereof, while FIG. 9 illustrates patterns connecting an IC and a heat generator part with each other and FIG. 10 is an enlarged sectional view showing the heat generator part.
Referring to FIGS. 7 and 8, a heat generator part 20 is provided on an end of an insulating substrate 21 along its longitudinal direction, while an IC 30 for driving the head is arranged on the other end. The heat generator part 20 is separated into respective dots. The heat generator part 20 and the IC 30 are electrically connected with each other by aluminum electrode patterns 31 every dot, as shown in FIG. 9. Illustration of the aluminum electrode patterns 31 is omitted in the blank part of FIG. 9.
The heat generator part 20 includes a glaze layer 22 which is formed on a surface of the insulating substrate 21 for serving as a heat storage layer as shown in FIG. 10, and a plurality of strip-shaped resistor layers 23 are formed on this glaze layer 22 in parallel with each other. These resistor layers 23 are provided thereon with a common electrode 24 and individual electrodes 25 consisting of metals, which are stacked and formed to be opposed to each other. A heat generation region 26 consisting of a resistor layer is provided between the common electrode 24 and the individual electrodes 25. The common electrode 24 is connected to an Ag common electrode 32 shown in FIG. 7, while the individual electrodes 25 are connected to the IC 30 through the aluminum electrode patterns 31 shown in FIG. 9. When a control signal is supplied to the IC 30, an electric signal is applied to the individual electrodes 25, so that the heat generator part 20 generates a heat signal for image formation by energization. A protective film 27 is formed on the heat generation region 26, the common electrode 24 and the individual electrodes 25 of the heat generator part 20, for covering and protecting the same.
When a printed medium is brought into contact with the heat generation region 26 and moved in the thermal print head, the heat signal must generally be transmitted from the heat generation region 26 to the printed medium which is in a state being pressed and moved in order to obtain an excellent printed image. Thus, adhesion between the heat generation region 26 and the printed medium is important. Under such circumstances, various structures of thermal print heads have been proposed in order to improve contactability of printed media with respect to heat generation regions.
For example, Japanese Patent Publication No. 7-10601 (1995) discloses a thermal print head in which common and individual electrodes are formed by metal wires of a multilayer structure thereby reducing the thicknesses of electrode parts adjacent to a heat generation region. In this thermal print head, a protective film once formed on the heat generation region is removed by etching in a constant amount from the heat generation region thereby attaining flatness of the protective film on this region, in order to improve the contact property of a printed medium with respect to the heat generation region.
Following the recent development of the semiconductor technique, on the other hand, there has also been proposed a thermal print head in which a heat generation resistor is prepared from polycrystalline silicon containing a constant amount of impurity. For example, Japanese Patent Publication No. 5-14618 (1993) discloses a thermal print head comprising a resistor layer consisting of polycrystalline silicon doped with an impurity element provided on a glaze layer which is formed on a ceramic substrate, and common and individual electrodes which are formed on the resistor layer to be opposed to each other.
The application field of the thermal print head is increasingly enlarged following development of the working technique, and a demand for application to a color printer capable of forming high-quality color images is particularly increased in recent years.
The so-called solid printing is relatively frequently employed in a head for such a color printer, due to its application. In the head for a color printer, therefore, a superior contact property of a printed medium with respect to a heat generation region is required as compared with a general head for monochromatic printing, while more sufficient electric energy must be supplied to common and individual electrodes. In the aforementioned head structure disclosed in Japanese Patent Publication No. 7-10601 (1995), however, the contact property with respect to the printed medium is improved by removing the protective film from the heat generation region by etching and attaining flatness of the protective film surface. Thus, an additional step for the etching is required and hence the steps are complicated, while the thickness of the protective film may be dispersed due to uneven etching.
In the thermal print head consisting of the resistor layer which is prepared from the polycrystalline silicon doped with an impurity described in Japanese Patent Publication No. 5-14618 (1993), on the other hand, portions of the heat generation region between the common and individual electrodes are still formed concave similarly to the structure shown in FIG. 10, and hence the printed medium cannot attain a sufficient contact property with respect to the heat generation region.
In the color printer, further, color irregularity may disadvantageously be conspicuous due to dispersion of heating values in the respective dots of the heat generator part in case of a high gradient of 256 gradations or the like, although such color irregularity is rather inconspicuous in case of 64 or 128 gradations, for example.